Ashworth Eleanor K, Anstöter Cate S, Verlet Jan R R, Bull James N
School of Chemistry, Norwich Research Park, University of East Anglia, Norwich NR4 7TJ, UK.
Department of Chemistry, Durham University, Durham, DH1 3LE, UK.
Phys Chem Chem Phys. 2021 Mar 18;23(10):5817-5823. doi: 10.1039/d1cp00261a.
Astrochemical modelling has proposed that 10% or more of interstellar carbon could be tied up as polycyclic aromatic hydrocarbon (PAH) molecules. Developing reliable models of the interstellar carbon lifecycle requires calibration data obtained through laboratory studies on relevant chemical and physical processes, including on the photo-induced and electron-induced dynamics of potential interstellar PAHs. Here, the excited state dynamics of the S1(ππ*) state of 2-naphthoxide are investigated using frequency-, angle-, and time-resolved photoelectron imaging. Frequency-resolved photoelectron spectra taken over the S1(ππ*) band reveal low electron kinetic energy structure consistent with an indirect, vibrational mode-specific electron detachment mechanism. Time-resolved photoelectron imaging using a pump photon energy tuned to the 0-0 transition of the S1(ππ*) band (hν = 2.70 eV) and a non-resonant probe photon provides the excited state autodetachment lifetime at τ = 130 ± 10 fs. There is no evidence for internal conversion to the ground electronic state or a dipole-bound state. These results imply that 2-naphthoxide has no resilience to photodestruction through the absorption of visible radiation resonant with the S1(ππ*) band, and that electron capture by the S1(ππ*) state, which is formally a shape resonance, is not a doorway state to a stable interstellar anion.
天体化学模型表明，星际碳中10%或更多可能以多环芳烃（PAH）分子的形式存在。要建立可靠的星际碳生命周期模型，需要通过对相关化学和物理过程进行实验室研究来获取校准数据，包括对潜在星际PAH的光致和电子诱导动力学的研究。在此，利用频率分辨、角度分辨和时间分辨光电子成像技术研究了2-萘氧负离子S1(ππ*)态的激发态动力学。在S1(ππ*)能带范围内采集的频率分辨光电子能谱揭示了低电子动能结构，这与一种间接的、特定振动模式的电子脱离机制相一致。使用调谐到S1(ππ*)能带0-0跃迁（hν = 2.70 eV）的泵浦光子能量和非共振探测光子进行的时间分辨光电子成像，得到激发态自脱离寿命为τ = 130 ± 10 fs。没有证据表明存在向基态或偶极束缚态的内转换。这些结果表明，2-萘氧负离子对通过吸收与S1(ππ*)能带共振的可见光而发生的光破坏没有抵抗力，并且S1(ππ*)态的电子俘获（形式上是一种形状共振）不是通向稳定星际阴离子的门态。
